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Mood stabilisers and everyday illness

Lithium and dehydration: why fluids and salt matter

Lithium levels are sensitive to how much water and sodium the body holds. Being dehydrated, whether from a stomach bug, a hot day or not eating and drinking, can raise the level and cause toxicity even when the dose has not changed. This guide explains the connection, the situations to watch for and the NHS advice on what to do.

Last checked: 2026-10-10

Key facts

At a glance.

  • Illness can raise levels

    The NHS says that feeling unwell while taking lithium can affect your medicine, and that infections and illnesses like colds and flu can make you dehydrated, which can affect the levels of lithium in your blood.

  • Call NHS 111 now

    The NHS advises calling 111 now if you have an illness causing severe diarrhoea, vomiting, a high temperature or sweating, a urinary tract infection, or are not eating and drinking much.

  • Sodium is the link

    NICE CKS advises that episodes of diarrhoea or vomiting, or any form of dehydration, will lead to sodium depletion and therefore increased plasma lithium levels.

  • Do not cut salt suddenly

    The NHS says it is important not to reduce your salt intake suddenly, and to talk to your doctor if you want to reduce the salt in your diet.

  • Alcohol and heat

    The NHS says drinking alcohol causes your body to lose water and is likely to make you dehydrated, especially in hot weather when you sweat more.

  • Dose reviews

    CKS says lithium dose should be reviewed as necessary in people with diarrhoea, intercurrent infection (especially if sweating profusely), vomiting, or following surgery.

How dehydration affects lithium

The NHS explains that infections and illnesses can make you dehydrated and that this can affect the levels of lithium in your blood. NICE CKS puts the practical consequence simply: diarrhoea, vomiting or any form of dehydration will lead to sodium depletion and therefore increased plasma lithium levels.

This matters because the level that helps and the level that harms are close together. A person stable on the same dose for years can therefore see the level rise during an illness, which is why the NHS asks you to call 111 when unwell.

Situations that can cause trouble

The NHS and CKS list several everyday situations in which fluid and sodium balance can shift. Recognising them in advance is the best protection.

  • Stomach bugs with vomiting or diarrhoea, and any illness with a high temperature or sweating.

  • Colds, flu and urinary tract infections, which the NHS says can make you dehydrated or affect your medicine.

  • Not eating and drinking much, for example during an illness or low appetite.

  • Hot weather or intense exercise, when the NHS says to drink plenty of fluids because you will sweat more.

  • Being immobile for long periods, or having a chest infection or pneumonia, which CKS lists as times to maintain fluid intake.

  • Following surgery, when CKS says the lithium dose should be reviewed.

  • Sudden or severe low-salt diets, which the NHS advises against starting without talking to a doctor.

What to do when you are unwell

The NHS lithium page is direct: if you have an illness that causes severe diarrhoea, vomiting, a high temperature or sweating, if you have a urinary tract infection, or if you are not eating and drinking much, call 111 now or use 111 online. The advice is to ask, rather than wait and see, because the clinician may want a lithium level and kidney tests.

Do not stop the lithium on your own without speaking to someone, because the NHS warns that stopping suddenly can make you unwell again quickly. That does not conflict with the advice to call 111: they can tell you what to do about the next dose.

Take small, frequent sips of fluid if you are being sick, as the NHS side-effects page suggests for dehydration, and keep your lithium record booklet and alert card to hand so a clinician can see your dose and recent levels.

Signs that lithium has become too high

The NHS says to call 999 or go to A&E now for symptoms that include stomach ache, feeling sick and diarrhoea, blurred vision, feeling very thirsty, needing to pee more than normal and lack of control over pee or poo, feeling lightheaded or drowsy, confusion and blackouts, shaking, muscle weakness, twitches, jerks or spasms of the face, tongue, eyes or neck, and difficulty speaking. These are signs of lithium toxicity, which the NHS calls an emergency.

You should stop taking lithium straight away in that situation, avoid driving yourself and take all your medicines with you to hospital. CKS notes there is no specific antidote and treatment in hospital is supportive, with levels rechecked regularly.

Everyday habits that help

People on lithium are advised by CKS to maintain their fluid intake, particularly after sweating, in hot climates, with a fever, or if immobile. The NHS suggests drinking plenty of fluids and moderating alcohol. Aiming for pale, regular pee is a sensible everyday guide, and the NHS lists peeing less than usual or dark, strong-smelling pee as signs of dehydration.

Keep appointments for blood tests; the NHS says weekly at the start and then every 3 to 6 months once the dose is steady. Mention holidays to hot countries, long-distance travel and any planned operations to the team in advance.

NHS and private care

The NHS says you are given a lithium treatment pack with a record booklet and alert card when you start. If you travel or use private care, bring the booklet so that levels and dates are visible. Out-of-hours services and A&E depend on you telling them that you take lithium, so say it early, and show the alert card.

Frequently asked

Questions people ask.

Quick, plain-English answers to the questions we hear most.

  • Why does being dehydrated raise lithium levels?

    NICE CKS explains that diarrhoea, vomiting or dehydration lead to sodium depletion and therefore increased plasma lithium levels.

  • When should I call NHS 111?

    The NHS says to call 111 now if you have an illness causing severe diarrhoea, vomiting, a high temperature or sweating, a UTI, or you are not eating and drinking much.

  • Should I skip my lithium if I am being sick?

    Do not decide alone. The NHS says not to stop lithium suddenly without speaking to a doctor, so phone 111 or your clinician for advice about the next dose.

  • Does hot weather matter?

    Yes. The NHS says to drink plenty of fluids, especially during intense exercise or hot weather, when you will sweat more.

  • Can I drink alcohol?

    The NHS says it is best not to drink too much because alcohol causes the body to lose water and is likely to cause dehydration.

  • Is a low-salt diet safe?

    The NHS says it is important not to reduce your salt intake suddenly and to talk to your doctor if you want to cut down on salt.

  • What if I forget a dose when ill?

    The NHS says that if it is less than 6 hours since the dose was due for tablets, take it; otherwise skip it. Never double up. Ask 111 if unsure.

  • What are the emergency signs?

    Confusion, blackouts, blurred vision, severe sickness and diarrhoea, shaking and muscle twitching. The NHS says call 999 or go to A&E.

Where this comes from

Sources and checking.

This guide is for information, not medical advice.

Your GP or consultant knows your history and can tell you which parts apply to you. If your symptoms are severe, rapidly worsening, or come with red-flag features, seek urgent medical care. Figures and availability change, so treat them as a guide and confirm with the provider.

Published 2026-10-10, next review 2027-10-10.

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